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Re: [Xen-devel] [semi-urgent Xen CS question] Re: git commit 9fd67b4ed0714ab718f1f9bd14c344af336a6df7 (x86-64: Give vvars their own page) breaks Xen PV guests (64-bit).

On 07/27/2011 09:33 PM, Andrew Lutomirski wrote:
> On Sandy Bridge, a null vsyscall takes 373 ns. Without
> VCGF_in_syscall, it's 457 ns. The change causes my little test app to
> get cs == __USER_CS.

Hm, 20% is more noticable than I would hope.  What about a regular syscall?

> I suspect that Sandy Bridge is just about the worst case.  syscall and
> sysret are amazingly fast on Sandy Bridge.

Yes, and one presumes it would only get worse.


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